The present invention pertains to a means for inferring the refractive index profile of the atmosphere and particularly to such means wherein an observed signal pattern is compared with a succession of hypothetical signal pattern models.
A precise knowledge of the refractive index profile of the atmosphere, which may comprise the atmospheric refractive index as a function of height, is important for accurately and reliably predicting the transmission of information through the atmosphere. For accurate transmission at VHF and higher frequency ranges (30 MHz-30 GHz) it is particularly important to have an accurate refractive index profile of the lower atmosphere (up to 20,000 feet). In addition, for certain applications it may be necessary to be able to quickly determine the "real" refractive index profile, which is the actual profile in a specified area at a specified point in time, rather than relying on a time-averaged "standard" refractive index profile, which may be quite different for the specified time. For example, for certain military applications the accuracy of the refractive index profile may be critical in planning operations where it is sought to evade hostile radar.
The calculation of the refractive index profile using prior art methods requires the use of balloons and radiosondes or aircraft carrying microwave refractometers. Each of these methods is relatively expensive, time-consuming (requiring a minimum of forty-five minutes) and requires substantial logistic support. Use of the radiosonde entails further disadvantage for military purposes since it transmits a signal which may be detected by a hostile receiver.
Further prior art means for determining a refractive index profile employs the principle of doppler shift in a satellite signal. However, such means require the use of a satellite transmitter having a higher stability than is presently used in satellite transmitters.